Acute 8 hour exposure of rats to 2450 MHz (CW) microwave radiation was performed at power levels that cause no increase in colonic temperature. There were no measurable changes in the mast cell response to compound 48/80 or to concanavalin A, nor were any morphological changes observed. There was no change in the thyroid axis, however, alterations were seen in serum corticosterone levels in the 10 mW/g (SAR) group. No change in hematocrits or erythrocyte number was observed and there were no changes in total leukocytes, or in the numbers of lymphocytes, neutrophils, and monocytes.